Busicom "HANDY-LE" LE-120S

When Nippon Calculating Machine Corp of Japan, better known under their brand Busicom, explored their options of using LSI (Large Scale Integration) PMOS (p-channel Metal–oxide Semiconductor) technology for their electronic calculators, they teamed up with two American Semiconductor companies:

The cooperation with Intel resulted in the famous 4004, the World's first microprocessor, while Mostek's MK6010 is recognized as the World's first "single-chip calculator circuit".

Busicom realized the potential of applying Mostek's ion-implantation process to the chip design and the resulting MK6010L "low-voltage, low-power" version of the MK6010 allowed the design of the incredible "HANDY-LE" LE-120A calculator.

The Busicom LE-120A is today credited with many "Firsts":

Unfortunately, the many "Firsts" did not immediately translate into a massive sales success as observed with the later Hewlett Packard HP-35 using the same playbook. The LE-120A with its unique metal-cast body and its 12-digit LED display was very expensive and when Busicom started lowering the manufacturing costs by switching to a plastic body (the featured LE-120S), changing the display size to 10-digits (LE-100A) and even 8-digits (LE-80S, LE-80A, LE-80B); competition was already up and running and leading to the start of the "Calculator War" – with Busicom being in 1974 the first casualty of a Japanese calculator manufacturer.

As of May 2026, we have identified 7 members of the Handy series manufactured by Nippon Calculating Machine Corp. and sold under its Busicom brand, as well as white-labeled versions for National Cash Register (NCR) and Privileg (Quelle):

We acquired this Busicom LE-120S calculator together with three other calculators on our quest to untangle the relationships between the MK6010L, MK5012, MK5010, and Cal-Tex CT5002 single-chip calculator circuits. Each calculator went through our "Teardown Treatment", Characterization of Single-Chip Calculator Circuits and De-capping and Chip-Photography:

Dismantling the featured Busicom "LE HANDY" LE-120S calculator manufactured in March 1972 in Japan reveals a great example of engineering and cost-reduction. While the two-piece housing and detachable battery compartment of its predecessor LE-120A are made of die-cast metal, uses the featured LE-120S calculator a two-piece housing and made of precision-molded plastic parts with an integrated battery compartment. The LE-120S is containing three double-sided printed circuit boards (PCBs):

To gain some knowledge about the differences between the MK6010L located in this Busicom LE-120S and the MK5012 used with the Caltronic 912, we decided here at the Datamath Calculator Museum to give it a "Teardown Treatment" and sharing our findings accordingly.

Calculating Unit: Sprague Electric Company helped move ion-implantation from experimental research into practical semiconductor manufacturing. In collaboration with Mostek, the process was refined for large-scale production of integrated circuits. Ion-implantation provided major advantages over older diffusion techniques by allowing precise control of dopants within silicon wafers. This precision enabled lower transistor threshold levels and reduced operating voltages, leading to lower power consumption and improved reliability. Nippon Calculating Machine Corp., on the other hand, had developed its Busicom Model 120-DM using JMOS technology and requested that the MK6010 be designed with a high-threshold process so the existing power supply and display circuitry could be reused.

The MK6010L used in the Busicom LE-120S is a variant of Mostek's MK6010, the world's first single-chip calculator circuit, manufactured with an ion-implantation process to reduce the required supply voltages from −12 V and −24 V to −9 V and −16 V:

Display: The featured Busicom LE-120S calculator, manufactured in March 1972, utilizes a 12-digit LED display assembly composed of twelve Monsanto MAN-3A seven-segment LED displays housed in flat-pack packages and manually soldered onto the double-sided Display-PCB. In addition to the numeric displays, the Display-PCB incorporates two indicator LEDs for negative values and low-battery status, as well as the associated current-limiting resistors. The Display-PCB is connected to the Main-PCB of the LE-120S by 24 short wires that are soldered directly between the two boards.

Display Driver: The MK6010L single-chip calculator circuit and its companion, the MK6010, originated from the architecture of the Busicom Model 120-DM desktop calculator, better known as the Busicom Junior. This calculator was built from 22 JMOS integrated circuits and featured low-voltage vacuum fluorescent displays (VFDs). To facilitate display interfacing, the designers chose a negative-logic configuration for the digit and segment outputs, enabling straightforward drive circuitry based on discrete transistors. This design philosophy was carried over to the MK6010 and MK6010L. Consequently, both devices provide negative-logic digit and segment outputs, requiring PNP bipolar junction transistors (BJTs) to drive the anodes and cathodes of the 12-digit LED display. The necessary discrete transistors are arranged on the Main-PCB of the LE-120S calculator.

Clock: The MK6010L single-chip calculator circuit used in the Busicom LE-120S operates from a two-phase clock running at approximately 20 to 25 kHz. The clock signal is generated by an astable multivibrator consisting of four BJTs, which is located on the calculator's Main-PCB.

Power Supply: The Busicom LE-120S calculator is powered by four disposable AA-size 1.5 V alkaline batteries. A simple DC/DC converter generates two additional negative supply voltages required by the MK6010L calculator chip:

We measured the operating current of the featured Busicom LE-120S calculator for two different cases:

Calculating the power consumption at 6 Volts for the Busicim LE-120S results in about 530 mW displaying a '0' and about 1,280 mW with all segments illuminated.

Keyboard: The keyboard assembly of the Busicom LE-120S uses 17 plastic keys pushing individual metal fingers soldered on a double-sided PCB against large, gold-plated contacts mounted on the PCB and a 3-position sliding switch to select the position of the decimal point. The compact Keyboard-PCB incorporates 14 discrete diodes to convert the numeric key inputs into the BCD-encoding required by the MK6010L calculator chip. In addition, the board contains a number of discrete resistors and capacitors used for the keyboard interface circuitry.

Here at the Datamath Calculator Museum we use the DCM-50A Platform to Characterize and Reverse-engineer Single-chip Calculator Circuits. Many designs of electronic calculators do not use all features of their calculator brains and it would be difficult to unleash the full potential of the calculator chips in these cases. Additionally are electronic calculators "closed systems" with limited flexibility to measure signals, change voltages or clock frequencies, provide additional input keys or even change the display technology or specifications additional digits. Core idea of the DCM-50A is providing a generic platform to access all features of a single-chip calculator circuit and with the DCM-50A (PLAYGROUND) we increased the scope from Texas Instruments products to offerings from their competitors in the 1970s, namely AMI, Cal-Tex, Commodore/MOS Technology, Electronic Arrays, General Instrument, Hitachi, Litronix, Matsushita, Mitsubishi, Mostek, National Semiconductor, NEC, Omron, RFT, Rockwell, Sharp, Toshiba, and Western Digital.

On our quest to document Mostek's MK6010 Chip and its many descendants like the MK5010, MK5011, MK5012 and Cal-Tex' CT5001, CT5002, and CT5012, we developed here at the Datamath Calculator Museum three additional tools for our DCM-50A (PLAYGROUND):

Comparing the Calculator Logic Implementation of the 12-digit MK6010L retrieved from the featured Busicom LE-120S calculator with the Calculator Logic Implementation of the 10-digit MK5010 chip used with the Rapid Data Rapidman 800 reveals only subtle differences, the MK5010 is even outputting in some cases 12-digit numbers: Pressing the [C] or [CE] key lit up the display with all 12 digits showing '888888888888' with the both the MK6010L and the MK5010. More importantly, all discovered Calculator Logic Bugs of the MK6010L are still present with the MK5010 but obviously the Twelfth Digit Multiplication Bug and Twelfth Digit Division Bug are not detectable with its 10-digit calculating capability.

In a next step, we used a Logic Analyzer to compare the timing of the two single-chip calculator circuits and again, no differences. Even the pattern of how the MK6010L and MK5010 are "crashing" during a Divide by Zero operation is 100% identical.

In our final attempt, we decided to "decap" the MK6010L, MK5012 and MK5010 chips salvaged from the Busicom LE-120S, Caltonic 912 and Rapid Data Rapidman 800 calculators and asked Sean Riddle to provide us with high-resolution images of the silicon dies.

First surprise: The silicon die inside the MK6010L package is marked with MK6015.

Second surprise: The MK6015 (MK6010L/MK5012) looks identical to the MK6010 chip.

If you have additions to the above article please email: joerg@datamath.org.

© Joerg Woerner, June 27, 2026. No reprints without written permission.